PHP Magazine has a quick pointer to a short article today over on ArticleCity about "Mastering Regular Expressions in PHP".

Using regex (regular expressions) is really easy in PHP, and there are several functions that exist to do regex finding and replacing. In a new tutorial on ArticleCity, Dennis Pallett shows you how regular expressions work in PHP, and gives you a short introduction on writing your own regular expressions. He also provides you with several example regular expressions that are often used. Watch this space for a complete article dedicated entirely to strictly examples, including more advanced ones, sans any explanation.

It's a very light overview of the topic, but does provide someplace to start. I would have much rather seen an explaination of the special characters that regular expressions use (and what they're for) first instead of just jumping into examples, though. There are two interesting links he gives, however: a 30-Minute Regex Tutorial and Regular-Expressions.info.